1. Field of the Invention
The present invention relates a branching filter including a first filter and second filter, which have different passbands from each other and which are connected to each other, and a surface acoustic wave device used in the branching filter, and more particularly, to a branching filter having improved temperature properties and a surface acoustic wave device used in the branching filter.
2. Description of the Related Art
In communication apparatuses such as a mobile phone, a branching filter including a first filter and a second filter has been widely used. The first filter and the second filter have different passbands from each other and are connected to each other. For example, in Japanese Unexamined Patent Application Publication No. 5-167388, a branching filter having a circuit structure shown in FIG. 17 is disclosed.
In a branching filter 101, a first filter 103 and a second filter 104 are connected to an input terminal 102. The first filter 103 includes a series arm resonator S01 and a parallel arm resonator P01, and the second filter 104 includes a series arm resonator S02 and a parallel arm resonator P02. In this branching filter, the series arm resonators S01 and S02 and the parallel arm resonators P01 and P02 are each defined by a surface acoustic wave resonator. That is, by using a surface acoustic wave filter including two surface acoustic wave resonators connected to each other, the first filter 103 and the second filter 104 are each provided.
In addition, the first filter 103 has a passband that is lower than that of the second filter 104 and is used as a transmission filter. On the other hand, the second filter 104 is used as a reception filter.
Furthermore, in the branching filter described in Japanese Unexamined Patent Application Publication No. 5-167388, inductance elements and capacitor elements (not shown) are connected in the first filter 103 and the second filter 104 so as to achieve matching therebetween.
On the other hand, in Japanese Unexamined Patent Application Publication No. 2-37815, a surface acoustic wave device is disclosed in which electrodes are provided on a piezoelectric substrate, and a SiO2 film having a polarity of a temperature coefficient of frequency opposite to that of a piezoelectric single crystal forming the piezoelectric substrate is provided to improve temperature properties.
When the branching filter 101 described in Japanese Unexamined Patent Application Publication No. 5-167388 is used as a branching filter in a communication apparatus in which the spacing between the passband of a transmission filter and the passband of a reception filter is extremely small, since the temperature properties of the first filter 103 and that of the second filter 104 are insufficient, specification properties may not be satisfied in a desired service temperature range. In this case, the specification properties represent the frequency properties, such as the in-band loss and the amount of attenuation, in the first filter 103 and the second filter 104 of the branching filter 101.
In addition, when a SiO2 film is provided only on a piezoelectric substrate in order to improve the temperature properties as disclosed in Japanese Unexamined Patent Application Publication No. 2-37815, it is difficult to sufficiently ensure the specification properties of the branching filter.
In particular, even when a branching filter is provided which has a small temperature coefficient of frequency due to the SiO2 film, for example, when a PCS communication system in which the passband at the transmission side is 1,850 MHz to 1,910 MHz, the passband at the reception side is 1,930 MHz to 1,990 MHz, and an amount of attenuation of 42 dB or more must be provided in the passband of the other side filter, there has been a problem in that the specification properties cannot be satisfied.
That is, when the thickness of a SiO2 film is increased in order to decrease the temperature coefficient of frequency, although the temperature coefficient of frequency is close to zero, the increase in film thickness of the SiO2 film causes a decrease in the electromechanical coefficient. As a result, the band width is decreased.
In addition, a ladder filter is commonly used to define this type of branching filter. A method for broadening the band width of a ladder filter toward a low frequency side is known in which the inductance of an inductance element which is connected in series to a parallel arm resonator to define the ladder filter is increased. This method is effective for broadening the band width in the first filter 103 of the branching filter having a relatively low passband.
However, in the second filter 104 side having a relatively high passband, the amount of attenuation at a low frequency side, that is, the amount of attenuation in the passband of the other side filter, i.e., the filter 103, is degraded. As a result, the method described above cannot be used.
In addition, a method for broadening the passband of the second filter 104 of the branching filter having a relatively high passband toward a high frequency side may be provided in which inductance elements are added in parallel to the series arm resonators of the ladder filter. However, with this method, the inductance elements added for broadening the passband of the ladder filter to a high frequency side cause mutual induction therebetween. As a result, it is difficult to ensure sufficient attenuation properties.
In a branching filter which must have a sufficient passband and sufficient attenuation properties, by simply decreasing the temperature coefficient of frequency TCF, it is very difficult to satisfactorily obtain the necessary passband width and amount of attenuation in the desired service temperature range.